1. Field of the Invention
The present invention relates to an infrared detecting device.
2. Description of the Prior Art
In general, an infrared detecting device includes the thermopile type, pyroelectric type, and bolometer type. The thermopile infrared detecting device includes the types shown in FIGS. 5 and 6, for example.
The difference between the thermopile infrared detecting devices shown in FIGS. 5 and 6 depends on the anisotropic etching method of the micromachining technique.
In the case of the infrared detecting device in FIG. 5, etching is started from the surface of a semiconductor substrate to form a diaphragm having a low thermal conductivity on the surface side and form a thermopile on the diaphragm.
In the case of the infrared detecting device in FIG. 6, the thermopile is formed on the surface of a semiconductor substrate, however etching is started from the back side of the semiconductor substrate.
The infrared detecting device in FIG. 5 is described below. As shown in FIG. 5, the thermopile infrared detecting device S1 is provided with a semiconductor substrate 101, a diaphragm 102 set on the semiconductor substrate 101 and having a low thermal conductivity, a thermopile 104 formed on the diaphragm 102, and a heat absorption film 105 formed through an insulation layer 103 located at the central portion. Thermopile 104 is formed by arranging a plurality of thermocouples 113 constituted by arranging p-type polysilicon 110 and n-type polysilicon 111, having the same width and thickness relative to each other, in parallel and electrically connecting them in series via aluminum wiring 112.
In order to improve the performance of the above infrared detecting device, it is required to increase the ratio of an output signal to a noise, that is, so-called SN ratio (Signal to Noise ratio). However, in the case of the above conventional infrared detecting device S1, the difference between electrical resistivities of the p-type polysilicon 110 and n-type polysilicon 111 of the thermocouple 113 and the difference between thermal conductivities of them are not considered at the time of designing the thermopile 104 and the width and thickness of each polysilicon are the same. Therefore, there is a problem in that it is difficult to obtain the device with high SN ratio and it is the subject to solve the above-mentioned problem.
The present invention is made in view of the aforementioned subject and it is an object to provide an infrared detecting device capable of setting sectional areas of p-type polysilicon and n-type polysilicon of a thermocouple to values suitable for improving the SN ratio.
The infrared detecting device according to the present invention is characterized by comprising a semiconductor substrate, a diaphragm set on the semiconductor substrate and having low thermal conductivity, a thermopile formed on the diaphragm by arranging a plurality of thermocouples composed of p-type polysilicon and n-type polysilicon in a row and electrically connecting the thermocouples with each other in series, and a heat absorption film formed on a detecting section of the thermopile through an insulation layer, wherein the p-type and n-type polysilicons are different in their sectional areas from each other at a position between a hot junction and a cold junction of each of the thermocouples forming the thermopile, in order to achieve the above subject on the prior art.
The infrared detecting device according to the first preferred embodiment of the present invention is constituted so that the p-type and n-type polysilicons are different in their average sectional areas from each other at the position between the hot and cold junctions of each of the thermocouples, and the infrared detecting device according to the second preferred embodiment of the present invention is characterized in that Ap/An ratio of the sectional area of the p-type polysilicon to that of the n-type polysilicon is in a range of {square root over ( )}(m) to {square root over ( )}(m)/n when a gap between the p-type and n-type polysilicons is constant and xcfx81p/xcfx81n ratio of electric resistivity of the polysilicons is defined as xe2x80x9cmxe2x80x9d and kp/kn ratio of thermal conductivity of the polysilicons is defined as xe2x80x9cnxe2x80x9d.
The infrared detecting device according to the third preferred embodiment of the present invention is characterized in that the p-type and n-type polysilicons are equivalent in their thickness, and different in their width from each other at the position between the hot and cold junctions of each of the thermocouples, the infrared detecting device according to the fourth preferred embodiment of the present invention is constituted so that the p-type and n-type polysilicons are equivalent in their width, and different in their thickness from each other at the position between the hot and cold junctions of each of the thermocouples, the infrared detecting device according to the fifth preferred embodiment of the present invention is constituted so that Wp/Wn ratio of the width of the p-type polysilicon to that of the n-type polysilicon is in a range of {square root over ( )}(m) to {square root over ( )}(m)/n when a gap between the p-type and n-type polysilicons is constant, the p-type and n-type polysilicons are equivalent in their thickness, xcfx81p/xcfx81n ratio of electric resistivity of the polysilicons is defined as xe2x80x9cmxe2x80x9d and kp/kn ratio of thermal conductivity of the polysilicons is defined as xe2x80x9cnxe2x80x9d, and the infrared detecting device according to the sixth preferred embodiment of the present invention is characterized in that tp/tn ratio of the thickness of the p-type polysilicon to that of the n-type polysilicon is in a range of {square root over ( )}(in) to {square root over ( )}(m)In when a gap between the p-type and n-type polysilicons is constant, the p-type and n-type polysilicons are equivalent in their width, xcfx81p/xcfx81n ratio of electric resistivity of the polysilicons is defined as xe2x80x9cmxe2x80x9d and kp/kn ratio of thermal conductivity of the polysilicons is defined as xe2x80x9cnxe2x80x9d.
The infrared detecting device according to the seventh preferred embodiment of the present invention is characterized in that ratio of width of polysilicon with high thermal conductivity to that of polysilicon with low thermal conductivity is maximized when the number of the thermocouples and a gap between the p-type and n-type polysilicons are constant, and the polysilicons are equivalent in their thickness.